1. Field of the Invention
The present invention relates to slip control system for a vehicle and, more particularly, to a slip control system for the vehicle, adapted to prevent a slip of the driven wheels on a road surface from becoming excessive by controlling torque to be applied to the driven wheels.
2. Description of Related Art
Prevention of a slip of the driven wheels from becoming excessive is effective in order to efficiently provide a propulsive force of the vehicle as well as to provide safety in terms of prevention of a spin of the vehicle body and so on. In order to prevent an excessive amount of a slip of the driven wheels, torque to be applied to the driven wheels as a cause of slipping may be reduced.
Slip control of this type is disclosed, for example, in U.S. Pat. Nos. 4,484,280 and 4,583,611 (corresponding to Japanese Patent Unexamined Application (kokai) Nos. 16,948/1983 and 56,662/1985, respectively). They disclose technique of reducing the torque applied to the driven wheels by braking the driven wheels by brakes as well as reducing the torque to be generated by the engine. More specifically, U.S. Pat. No. 4,484,280 (corresponding to Japanese Patent Unexamined Application (kokai) No. 16,948/1983) discloses the technique of reducing the torque to be applied to the driven wheels only by braking the driven wheels when a slip of the driven wheels is small and by reducing the torque generated by the engine, on top of braking the driven wheels, when a slip of the driven wheels becomes larger. U.S. Pat. No. 4,583,611 (corresponding to Japanese Patent Unexamined Publication (kokai) No. 56,662/1985) discloses the technique in which, when one of the left-hand and right-hand driven wheels is larger in slippage than the other, only the driven wheel having a larger slip value is braked and, when both of the driven wheels are larger in slippage, the two driven wheels are braked while the torque to be generated by the engine is reduced. In summary, the technique as disclosed in the two prior patent publications as described hereinabove uses the braking of the driven wheel or wheels by the brake or brakes as a main means and the reduction in the torque generated by the engine as an auxiliary means.
As a result of experiments, it has been found that, for a slip control system of the type reducing the torque to be applied to the driven wheel by braking when a slip ratio of the driven wheel on pavement reaches a value that is larger than a predetermined value, the vehicle becomes likely to be unstable at the time of driving at high speed. This seems to arise because a posture of the vehicle gets unstable due to a close coupling of the brake with the driven wheels if the braking force would not be controlled in an appropriate manner, although a response is excellent.